2,4-Dichlorophenoxyacetic acid (2,4-D) is a herbicide used primarily to control dicotyledonous weeds. 2,4-D is broken down in soil by a variety of microorganisms, including Alcaligenes eutrophus. A gene (tfdA) has been isolated from strains of A. eutrophus which encodes the first enzyme in the 2,4-D degradation pathway of these bacteria. This enzyme is a dioxygenase which catalyzes the conversion of 2,4-D to 2,4-dichlorophenol (DCP).
Transgenic tobacco plants, cotton plants, and hardwood trees containing the tfdA gene have been reported to have increased tolerance to 2,4-D. Streber et al., Bio/Technology, 7, 811-816 (1989); Lyon et al., Plant Molec. Biol., 13, 533-540 (1989); Bayley et al., Theor. Appl. Genet., 83, 645-649 (1992); Llewellyn and Last, in Herbicide-Resistant Crops Chapter 10, pages 159-174 (Duke, ed., CRC Lewis Publishers 1996); Last and Llewellyn, Weed Science, 47, 401-404 (1999); U.S. Pat. Nos. 5,608,147, 6,100,446, and 6,153,401; PCT application WO 95/18862. However, 2,4-D-tolerant transgenic cotton has been reported to have significantly reduced growth rates when sprayed with 2,4-D at levels that might be encountered in agricultural situations. Last and Llewellyn, Weed Science, 47, 401-404 (1999).
Increases in growth and yields have been reported to occur from applications of sublethal concentrations of 2,4-D and other herbicides to plants sensitive to them. See, Allender, J. Plant Nutrition., 20,69-80 (1997); Moffett et al., Crop Sci., 20,747-750 (1980); Wiedman and Appleby, Weed Res., 12, 65-74 (1972); Miller et al., Crop Sci., 2, 114-116 (1962); McIlrath and Ergle, Botanical Gazette, 461-467 (1953); McIlrath and Ergle, Plant Physiol., 693-702 (1952). However, the growth stimulation is usually small, highly variable, transitory, and difficult to reproduce in the field. Allender et al., J. Plant Nutrition., 20, 81-95 (1997).